In wavelength division multiplexing (WDM) optical communication systems a single optical fiber may be used to carry multiple optical signals. The multiple optical signals are multiplexed to form a multiplexed signal or WDM signal with each of the multiple signals being modulated on a separate channel. Each channel may be at an associated wavelength that is separated from adjacent channels by a defined channel spacing, e.g. according to a channel plan established by the International Telecommunications Union (ITU). The specific range of data channel wavelengths designed to be transmitted on the system is known as the system bandwidth. Systems may utilize their system bandwidth to carry a desired number of channels with a desired modulation format and bit rate.
One measure of the quality of the signals transmitted in a WDM optical communication system is optical signal-to-noise ratio (OSNR). OSNR is the ratio of the channel power to noise power the system bandwidth or in a portion of the system bandwidth, and can be used to predict the bit error rate (BER) of a channel. Conventionally, OSNR measurements for optical communication systems have been performed when the system is out-of-service, e.g. at initial installation of the system before transmission of optical signals.
Out-of-service OSNR testing has the significant drawback that transmission of data signals is terminated to perform the testing. Also, using OSNR measurements taken in an out-of-service mode at the initial installation of the system fails to account for changes in system performance during the life of the system. For example, as amplifiers within the system age the amplified spontaneous emission (ASE) noise generated by the amplifiers may change, which, in turn, results in a change of the actual system OSNR. Accordingly, attempts have been made to provide for in-service OSNR testing. Unfortunately, known systems and methods for performing in-service OSNR testing have not provided sufficient accuracy in determining OSNR.